What is this black line in my rendered donut?
It looks like You are following the tutorial for beginners made by BlenderGuru aka Andrew Price, Hassan, right? If I remember it right he duplicates the top half of the donut and make it bigger and then use proportional falloff to make that random chocolate edge.

Because of duplication of a top half part of a donut there is a big chance of overlapping vertices so to solve this, please select the donut (complete one) and go to EditMode and DEACTIVATE the "Limit selection to visible" (bottom green circle on the image) to make the hidden vertices visible. Once done please select the top vertices with the B key (WITHOUT the middle ones - marked RED) and delete them with the X key choosing VERTICES as shown on the picture - done!

The vertices which needs to be deleted are selected below ... You dont need that top part of a donut anymore cause You have the chocolate object instead now ;). Hope it helps buddy.